This is what the SPF number of your sun cream actually means

Obviously I know SPF stands for 'sun protection factor', but I thought the 15, 30, 50 and so on was just a random numerical scale much like tennis scoring, the greater the number only generally indicating a heavier level of coverage.

But it turns out I was nowhere near the truth. There is a rational reason why the SPF numbers are what they are: they indicate how many multiples of time you're safe to be out in the sun before you burn.

And it makes total sense. SPF 15 will give you 15 times the number of minutes before you burn that you'd have with no sunscreen protection at all.

Sincere apologies if you've known this your whole life and I'm here looking like a complete plonker because I wasn't aware, but I'm one of at least 3 people currently surrounding me who also didn't know, so it would've been wrong not to share the knowledge.